The art of surveying involves determination of unknown positions, surfaces or volumes of objects using measurements of angles and distances. In order to make these measurements, a surveying instrument frequently comprises an electronic distance measuring device (EDM) that may be integrated in a so-called total station. A distance measuring total station combines electronic, optical and computer techniques and is furthermore provided with a computer or control unit with writable information for controlling the measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail.
A tracker system or unit of an optical total station normally focuses a light signal from a target located at an object, either a reflection of an incident measuring beam comprising optical radiation from the target or from an active device on the target, onto a detector. A servo system of the total station may move or turn the total station based on the signal from the detector.
Further, when performing distance measuring or surveying tasks using a distance measuring total station at a work site, it is often desirable to measure a surface or volume of an object being present on the work site. In such a work site, it may, for example, be desirable to scan a surface of an object, for example, a wall of a building to obtain an image of the wall. For such applications, a distance measuring total station may be implemented as a geodetic scanner for determining the appearance of the object or target based on the measurements of distances to positions of interest at the surface of the target. Such a scanner may register the surface or volume of the target or even monitor changes in a scene.
The geodetic scanner is often set up at a certain position at the work site, from which position the surface or volume of the target that is to be scanned is visible. For scanning surfaces or volumes of the target or of another target that are not visible from the position at which the geodetic scanner is set up, the geodetic scanner has to be moved to another position at which visibility of the surfaces or volumes desired to be scanned is attained. This process may be cumbersome and time-consuming, e.g., due to the labor involved in moving the geodetic scanner and in setting up the geodetic scanner once it has been positioned at a new position.
For overcoming these disadvantages, mobile scanners are available that can be operated hand-held. In other words, a user can carry the mobile scanner to a suitable position at which the surfaces or volumes desired to be scanned are visible, and at that position operate the scanner so as to perform scanning of the surfaces or volumes. In order to know where the light beam of the mobile scanner is aiming during a measurement session, e.g., in relation to some fixed coordinate system such as a coordinate system related to another measuring instrument at the work site, position and orientation of the mobile scanner must be monitored during the measurement session, e.g., in relation to the fixed coordinate system.
EP 1200853 A1 discloses a method for determining the orientation of an object onto which a reflector is arranged, which reflector has a non-adjustable or adjustable orientation with respect to the object. A laser tracker is sending out a laser beam impinging on the reflector. The direction and path length of the laser beam and the angle of incidence of the laser beam into the reflector and/or the reflector orientation relative to the object are measured. Using the measured data, the orientation and position of the object are determined. For measuring the angle of incidence, a position sensitive diode position sensor, arranged in a plane perpendicular to the optical axis of the reflector, is arranged behind the reflector such that the beam part penetrating through the reflector impinges thereon. According to EP 1200853 A1, the position of the light spot detected by the sensor is directly dependent on the angle of incidence of the laser beam into the reflector.
U.S. Pat. No. 7,312,862 B2 discloses a measurement system for determining six degrees of freedom of a reflector or of an object on which the reflector is arranged. The measurement system comprises a laser tracker equipped with a distance measuring apparatus, the laser tracker directing a laser beam (measurement beam) to the reflector and following or tracking the reflector, when moving, and detecting the direction of the measurement beam relatively to the tracker. The measurement system also comprises an optically detectable addition element arranged on the object for determining rotation of the reflector or of the object about the reflector axis or about the measurement beam. The addition element consists of two light points, which are passive markings, reflectors, active light sources or points produced by light beams directed against the object. The two light points are imaged in an imaging plane with a camera that is stationary with respect to the measurement beam during measurement. Each of the light points is identified in an image taken by the camera, or the light points are synchronized with the camera, such that the light points appear one after the other on consecutively recorded images, and a line connecting the two light points is determined from the images. According to U.S. Pat. No. 7,312,862 B2, an angle between the line and a likewise directed reference line is a measure of the roll angle of the reflector or the object.
Such arrangements for determining orientation of an object may provide only limited angular accuracy and/or angular resolution that can be insufficient in some applications. In other words, such arrangements for determining orientation of an object may not be able to determine orientation of the object with the required degree of accuracy.
Such arrangements for determining orientation of an object may provide only a limited range of operation with regards to the distance between the object and the measuring apparatus effectuating the determination of orientation of the object with regards to some applications. That is, the maximum distance between the object and the measuring apparatus effectuating the determination of orientation of the object may be relative small.